The present invention relates to polymer films. Specifically, the present invention relates to coated multilayer polyethylene films having improved sealability and barrier properties.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages including melt film formation, quenching and windup. For a general description of these and other processes associated with film-making, see K. R. Osborn and W. A. Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.
When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the direction of stretch, but it is weak in the other direction, i.e., across the stretch, often splitting or tearing into fibers (fibrillating) when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions, in which the crystallites are sheetlike rather than fibrillar. These biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.
It is technically quite difficult to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use apparatus that stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or xe2x80x9cmachine directionxe2x80x9d (MD), and then in the direction perpendicular to the machine direction, i.e., the lateral or xe2x80x9ctransverse directionxe2x80x9d (TD).
The degree to which a film can be oriented is dependent upon the polymer from which it is made. Polypropylene, polyethylene terephthalate (PET), and nylon are highly crystalline polymers that are readily heat stabilized to form dimensionally stable films. These films are well known to be capable of being biaxially stretched to many times the dimensions in which they are originally cast (e.g., 5xc3x97 by 8xc3x97 or more for polypropylene).
High density polyethylene (HDPE), however, exhibits even higher crystallinity (e.g., about 80-95%) relative to polypropylene (e.g., about 70%). As a result, HDPE films are generally more difficult to biaxially orient than polypropylene films. U.S. Pat. Nos. 4,870,122 and 4,916,025 describe imbalanced biaxially oriented HDPE-containing films that are MD oriented up to about two times, and TD oriented at least six times. This method produces a film that tears relatively easily in the transverse direction. Multi-layer films prepared according to this method are also disclosed in U.S. Pat. Nos. 5,302,442, 5,500,283 and 5,527,608, which are incorporated herein by reference.
U.S. Pat. No. 5,885,721, which is incorporated herein by reference, also discloses a multi-layer film having high biaxial orientation. In particular, this patent discloses a film including a HDPE substrate, at least one outer layer of a propylene copolymer, and at least one outer layer of an adhesion promoter material positioned between the outer layer and the HDPE substrate. The film is high biaxially oriented, being stretched in the machine direction to a degree of from about 5:1 to about 8:1, preferably from about 6:1 to about 7:1, and stretched in the transverse direction to a degree of from about 6:1 to about 15:1, preferably from about 9:1 to about 13:1.
The film-making process can also include coating a film to impart superior characteristics to the film and methods of coating are well known in the art. Most known methods provide for coating a film after it has been biaxially oriented.
Polyethylene films produced by processes known in the art have been widely used for structures such as grocery sacks or bread wrappers, but generally, they have been inadequate for packaging products such as snack foods and the like. Further, polyethylene films, in some applications, have lacked sufficient flavor and aroma barrier properties. Also, polyethylene films have lacked the ability to provide seal characteristics which are often required by food packagers.
Accordingly, it is one of the purposes of this invention, among others, to provide coated multilayer polyethylene films having improved sealability, oxygen barrier properties and flavor and aroma barrier properties, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film.
It has now been discovered that these and other purposes can be achieved by the present invention, which provides for coated multilayer polyethylene films having improved sealability and barrier properties.
The present invention provides for a multilayer polyethylene film including a substrate and at least one coating selected from the group including polyvinylidene chloride (PVdC), low temperature seal coating (LTSC), polyvinyl alcohol polymer (PVOH) and acrylic coating. The substrate includes a high density polyethylene (HDPE) base layer and at least a first skin layer adhered to a first side of the base layer and substantially coextensive therewith. The substrate has been oriented in the machine direction to a degree of from about 5:1 to about 8:1 and in the transverse direction to a degree of from about 6:1 to about 15:1. The HDPE of the base layer preferably has a density of not less than about 0.940. (Density (d) is expressed as g/cm3.)
In a preferred embodiment, the coating is at least 1 to 25 wt % of the multilayer film and the substrate is at least 75 wt % of the multilayer film. In another preferred embodiment, the coating is 10 to 15 wt % of the multilayer film and the substrate is 85 to 90 wt % of the multilayer film. In addition, the HDPE of the base layer has a density of at least about 0.958.
Another preferred embodiment of the present invention also provides for a multilayer polyethylene film including a substrate and at least one coating selected from the group including PVdC, LTSC, PVOH and acrylic coating. The substrate includes a HDPE base layer, at least a first skin layer adhered to a first side of the base layer and substantially coextensive therewith and a second skin layer adhered to a second side of the base layer and substantially coextensive therewith. The substrate has been oriented in the machine direction to a degree of from about 5:1 to about 8:1 and in the transverse direction to a degree of from about 6:1 to about 15:1.
Another preferred embodiment of the present invention also provides for a multilayer polyethylene film including a substrate and at least one coating selected from the group including PVdC, LTSC, PVOH and acrylic coating. The substrate has at least three layers including a HDPE base layer, a tie layer coextensive to the base layer and a first skin layer adhered to the tie layer and substantially coextensive therewith. The substrate has been oriented in the machine direction to a degree of from about 5:1 to about 8:1 and in the transverse direction to a degree of from about 6:1 to about 15:1.
Preferably, the substrate also includes a first tie layer interposed between the base layer and the first skin layer which is coextensive with each of the base layer and the first skin layer, and a second tie layer interposed between the base layer and the second skin layer which is coextensive with each of the base layer and the second skin layer.
Preferred embodiments of polyethylene films according to the present invention can include two or more coatings selected from the group including PVdC, LTSC, PVOH and acrylic coating. For example, a five-layer substrate as provided above can include on both the first skin layer and the second skin layer a coating selected from the group including PVdC, LTSC, PVOH and acrylic coating.
Preferred embodiments of polyethylene films according to the present invention provide for the addition of a cavitating agent to the substrate before the substrate is oriented. Preferably, the cavitating agent is selected from a group consisting of calcium carbonate (CaCO3), titanium oxide (TiO2) and polystyrene.
The present invention provides for multilayer polyethylene films with improved sealability, oxygen barrier properties and flavor/aroma barrier properties. The films also have excellent optics. These properties make these films an excellent alternative to films currently used in food packaging.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.